Patent Document 1 describes a technique of appending a seal member to the periphery of a wafer thereby leaving a thick peripheral portion as it is for preventing cracking or bending of the wafer upon etching the wafer rear face for lowering the on resistance of a vertical type MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
Patent Document 2 describes a technique for improving the strength and the durability of the wafer in a vertical power device by forming a rim-shaped hard mask with a silicon nitride film or the like at the periphery of a wafer rear face and conducting etching using the same as a mask, thereby leaving a thick peripheral portion as it is.
Patent Document 3 discloses a technique of leaving a thick periphery of a wafer in a ring shape as it is for ensuring the strength of the wafer upon reducing the film thickness of a silicon wafer in the process for producing a semiconductor device, and filling a silicon oxide film, a silicon nitride film, etc. in a lattice-shape at the rear face of the wafer previously and etching the silicon portion so as to leave them thereby reducing the thickness.
Patent Document 4 and Patent Document 5 disclose techniques of forming a patterned silicon oxide film, a silicon nitride film, etc. for relieving warping of a wafer.
Patent Document 6 and Patent Document 7 disclose a technique of using a silicon nitride film pattern as an etching mask and removing the silicon nitride film pattern after etching upon reducing the thickness of a wafer.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2003-243356    [Patent Document 2] Japanese Unexamined Patent Publication No. 2001-44088    [Patent Document 3] Japanese Unexamined Patent Publication No. 2007-317950    [Patent Document 4] Japanese Unexamined Patent Publication No. 2005-322839    [Patent Document 5] US Unexamined Patent Publication No. 2005-0253235    [Patent Document 6] Japanese Unexamined Patent Publication No. 2007-335659    [Patent Document 7] Japanese Unexamined Patent Publication No. 2008-53595